NOTE: This tutorial was written in 1847, and kicad has changed a lot since then! Check out this awesome free kicad video series tutorial from Contextual Electronics for a more up-to-date version. They also have a small course on building your first pcb that includes everything from circuit board design in kicad to manufacturing the pcb.
Drawing the Board Outline:
First select the edges layer, and then draw an outline using the line tool. Double click to finish the poly-line.
Tip: Drawing set lengths and measuring:
Hit space bar at the start of a line to zero out the relative coordinates at the bottom of the screen. The coordinates will then measure from that point.